1. Field of the Invention
This invention relates to a constant-voltage circuit for generating a stable constant voltage even if an input voltage is changed by various external effects.
2. Description of the Prior Art
Generally, it is desirable to derive a constant-voltage circuit which generates a stable constant voltage even if an input voltage is changed, and which itself consumes little current.
A prior constant-voltage circuit is showed in FIG. 1. In this circuit, an input voltage is supplied to input terminals 1 and 2, and a constant output voltage is derived from output terminals 3 and 4. NPN transistor 5 is connected in series between the terminals 1 and 3, with the collector NPN transistor 5 connected to the input terminal 1, and the emitter thereof connected to the output terminal 3. A collector of NPN transistor 6 is connected to a base of the transistor 5, and an emitter of transistor 6 is connected through constant-voltage diodes 7 and 8 to a ground point. The collector of the transistor 6 is also connected to the input terminal 1 through a resistor 9. Resistors 10 and 11 are connected in series between the output terminals 3 and 4. The connecting point of resistors 10 and 11 is connected to a base of the transistor 6, and a negative feedback loop is constructed. A load 12 is connected between the output terminals 3 and 5. The base-emitter voltage V.sub.BE of the transistors 5 and 6 is assumed to equal the anode-cathode voltage of diodes 7 and 8. Generally, the circuit is constructed on one semiconductor chip. In this case, diodes 7 and 8 are constructed by connecting a collector and a base of a transistor in common. So, the anode-cathode voltage of the diode is equal to the base-emitter voltage V.sub.BE of the transistor.
The output voltage V.sub.O in this circuit is given by ##EQU1## where V.sub.BE is a base-emitter voltage of a transistor or an anode-cathode voltage of the diode, R.sub.10 is the value of the resistor 10, and R.sub.11 is the value of the resistor 11. The value of the base-emitter voltage V.sub.BE of the transistor in the equation (1) varies in response to the flowing current. In this circuit, the value of V.sub.BE is determined by the current I.sub.1 in the resistor 9. The value of I.sub.1 in the resistor 9 is given by ##EQU2## where R.sub.9 is the value of resistor 9. Thus, in this circuit the value of I severely varies in response to the fluctuation of the input voltage V.sub.i. So the output voltage V.sub.O likewise severely varies in response to the input voltage V.sub.i from the equation (1).
Now it is assumed that the constant-voltage output V.sub.O of 5 volts is derived by the input voltage V.sub.i of 8 volts, and the input voltage V.sub.i is changed from 8 volts to 15 volts. The fluctuation of the output voltage in this case is calculated as follows. Now the standard voltage value of the base-emitter voltage V.sub.BE of the transistor is about 0.7 volts. So the value of (R.sub.10 +R.sub.11)/R.sub.11 must be equal to 2.38 from the equation (1). And the current I.sub.1 of the equation (2) is 2.3 volts/R.sub.9.
Next, the case that the input voltage V.sub.i is changed to fifteen volts is calculated. The current I.sub.1 is 9.3 volts/R.sub.9 from the equation (2). So the current in the diodes 7 and 8 and the collector current in the transistor 6 is changed from 2.3 volts/R.sub.9 to 9.3 volts/R.sub.9.
Generally, the fluctuation of the voltage V.sub.BE is calculated from an equation of diffusion potential as follows: ##EQU3## Where I.sub.O is the first value of the collector current of the transistor, k is Boltzman constant, q is charge value of an electron, and T is absolute temperature. In this case, the fluctuation .DELTA.V.sub.BE of the base-emitter voltage V.sub.BE is equal to 36.32 mV at room temperature. So the fluctuation of the output voltage V.sub.O becomes 259 mV from the equation (1).
As the base-emitter voltage V.sub.BE is 0.736 V (V.sub.BE =0.7 V+0.036 V), in case that the input voltage V.sub.i is 15 V, the fluctuation .DELTA.V.sub.BE of the base-emitter voltage V.sub.BE becomes 36.32 mV from the equation (1) at room temperature. Thus the fluctuation of the input voltage V.sub.i influences the output voltage V.sub.O largely.
The current I.sub.1 in the resistor 9 becomes larger from the equation (2) so that the difference between the input voltage V.sub.i and the output voltage V.sub.o becomes larger. As this current flows to ground through the transistor 6, the current consumed in the circuit is increased unnecessarily.
Another prior art constant voltage circuit is shown in FIG. 2, where the resistor 9 is replaced by the transistor 13 which operates as a constant-current source. So, as the value of the base-emitter voltage V.sub.BE is kept nearly constant, the output voltage V.sub.O is kept more constant. But this circuit has disadvantages, as follows. This circuit requires a special starting circuit because all transistors 5, 6, 13, 14 and 15 are off when the input voltage V.sub.i starts from zero volt such that the circuit of the transistors 5, 6, 13, 14 and 15 constitute a positive feedback circuit. A resistor 16, a diode 17, a transistor 18 and a resistor 19 constitute a circuit for starting. As the current in the transistor 6, the diodes 7 and 8 are determined by V.sub.BE /R.sub.19 the current is kept nearly constant. However, this current flows to the ground and represents lost power. Thus, although this circuit producers a constant voltage, the circuit is highly complex and wastefully consumes larger amounts of current.